FIG. 1 shows a cross-section of a conventional semiconductor laser device having a so-called window structure, which can operate to provide high output power. This semiconductor laser device has an active layer of a superlattice structure. In FIG. 1, successively disposed on an N-type GaAs substrate 1 are an N-type AlGaAs lower cladding layer 2, a GaAs/AlGaAs superlattice-structure active layer 3, a P-type AlGaAs upper cladding layer 4, and a P-type GaAs contact layer 5, with the N-type AlGaAs lower cladding layer being disposed on the substrate 1. Zn is diffused into the thus formed structure from the surface at opposite ends of the contact layer 5 to form Zn diffusion regions 6. The Zn diffusion disorders opposite end portions of the superlattice structure of the active layer 3 to thereby form disordered regions 7, 7'. The disordered regions 7 and 7' provide laser resonator end surfaces 8 and 8'.
The semiconductor laser device of FIG. 1 may be fabricated by a process such as one shown in FIGS. 2 and 3. As shown in FIG. 2, the N-type AlGaAs lower cladding layer 2, the GaAs/AlGaAs superlattice active layer 3, the P-type AlGaAs upper cladding layer 4, and the P-type GaAs contact layer 5 are sequentially grown in the named order on the N-type GaAs substrate 1. Next, a film of, for example, Si.sub.3 N.sub.4 is deposited over the P-type GaAs contact layer 5, and photolithographic techniques are used to form a mask 9 with a desired pattern from the Si.sub.3 N.sub.4 film.
Then, Zn is diffused into the structure through the openings in the mask 9 to form disordered regions 60. Then, the mask 9 is removed, and the structure is cleaved in the thickness direction at the center of each disordered region 60. Thus, semiconductor laser devices such as the one shown in FIG. 1 result.
FIGS. 4(a) and 4(b) illustrate another method of selectively disordering a GaAs/AlGaAs superlattice structure, which is shown in J. Appl. Phys. 64(7), Oct. 1, 1988, pages 3439-3444. FIG. 4(b) is an enlarged cross-sectional view of a portion of an area A shown in FIG. 4(a). A semiconductor laser chip 10 having a superlattice structure therein is placed on a computer-controlled X-Y table (not shown). The semiconductor laser chip 10 has an Si.sub.3 N.sub.4 coating 13 on its surface. An Ar laser beam 40 is directed via a mirror 30 and a condenser lens 20 to a predetermined location on the surface of the semiconductor laser chip 10 with the Si.sub.3 N.sub.4 coating 13 thereon. Ar laser beam energy is absorbed by that portion of the semiconductor laser chip 10 where the Ar laser beam impinges, so that that portion is locally heated to a high temperature, which causes Si in the Si.sub.3 N.sub.4 coating 13 to be diffused into a superlattice layer 11. This, in turn, promotes interdiffusion of GaAs and AlGaAs which constitute the superlattice layer 11, whereby a disordered region 12 is formed. By moving the X-Y table on which the semiconductor laser chip 10 is mounted, a disordered region can be formed in any desired pattern. By cleaving the semiconductor laser chip 10 in its thickness direction at the center of the disordered region 12 shown in FIG. 4(b), a semiconductor laser device similar to the one shown in FIG. 1 is provided.
The semiconductor laser device shown in FIG. 1 is a typical example of semiconductor laser devices which may be fabricated by the techniques shown in FIGS. 2 and 3 and FIGS. 4(a) and 4(b). In FIG. 1, the P-type GaAs contact layer 5 and the N-type GaAs substrate 1 are connected across an operating power supply to cause current to flow therebetween. This causes light to be generated in the active layer 3, and oscillations occur in a waveguide formed by the active layer 3 and the two opposing end surfaces 8 and 8', which operates as a resonator. In the regions 7 and 7' where the GaAs/AlGaAs superlattice structure of the active layer 3 is disordered due to the Zn diffusion, the bandgap is greater than that of GaAs, so that laser light generated in the active layer 3 is emitted without being absorbed. Accordingly, the heat generated in the disordered regions 7 and 7' is significantly reduced, and, therefore, the laser device can be operated to provide high output power.
In the conventional method shown in FIGS. 2 and 3, Zn is diffused from the surface of the P-type GaAs contact layer 5 in the thickness direction to reach the N-type AlGaAs lower cladding layer 2. According to this conventional method, because Zn has to be diffused relatively deeply thermal treatment for a long time is required. Such a long thermal treatment may also disorder superlattice structure portions into which no Zn has been diffused. Furthermore, the width a of the disordered regions 7, 7' (see FIGS. 1 and 3) cannot be made smaller than about one-half the depth of diffusion. A larger width of the disordered regions adversely affects various laser characteristics. For example, it may increase laser beam astigmatism. In addition, in order to fabricate discrete laser devices from the semiconductor laser chip after the Zn diffusion, it is necessary to cleave the chip at the center of the disordered regions 60, 60 (FIG. 2). If the cleavage at the center of the disordered regions fails, a desired window structure cannot be obtained, and, therefore, this method provides a low yield.
In the method shown in FIG. 4, it is necessary to precisely position the region to be disordered relative to the Ar laser beam by means of the X-Y table. However, the positional adjustment is difficult, and sufficient precision is hardly attained. Furthermore, it is difficult to establish a precise distance between the condenser lens 20 and the semiconductor chip 10.
An object of the present invention is to provide a method of fabricating a semiconductor laser device free of the above-stated drawbacks of the conventional methods. According to the present invention, portions of an active layer having a semiconductor superlattice structure are disordered in a self-aligning manner.